The present invention relates to electrical devices, and more particularly, to cathode electrodes adapted for use in electrolytic, dielectric oxide film-forming metal anode capacitors and the like.
While the present invention will be discussed hereinafter with reference to use in electrolytic capacitors, it is to be understood that the invention may be utilized in a variety of other electrical devices where a cathode electrode is required.
Electrolytic capacitors of the sintered dielectric oxide film-forming metal anode type generally consist of a cathode electrode, an electrically conductive electrolyte and a porous anode with a dielectric oxide film formed thereon. Typically, the cathode electrode is composed of silver, an alloy of silver, copper, or an alloy of copper to impart high electrical conductivity and corrosion resistance, the electrolyte is an aqueous solution of an inorganic acid, and the anode is composed of a film-forming metal such as tantalum, aluminum, or niobium, preferably tantalum.
In most situations, the cathode electrode also functions as the case or housing for the electrolytic capacitor by surrounding and retaining both the anode and the electrolyte. Present practice is to utilize silver as a main constituent of the cathode electrode because of the desirable electrical and chemical properties of silver. Due to the relatively high cost of silver, it would be advantageous to use copper or copper alloys for the cathode electrode while still retaining the performance characteristics of silver-based cathode electrodes.
The anode in this type of electrolytic capacitor is generally formed by pressing powders of the particular film-forming metal into the desired shape and then sintering the pressed powder. The resultant sintered anode is characterized as having a myriad of interconnecting void areas and therefore has a very large surface area per unit of volume which contributes greatly to the capacitance of the device in which it is utilized. A dielectric oxide film of the metal is then formed over the anode, typically by an electrolytic anodization process.
While the anode exhibits a relatively large capacitance, the interface between the cathode electrode and the electrolyte has an inherent charge separation due to polarization and thus exhibits a cathodic capacitance. This cathodic capacitance due to polarization may result from the formation of an asymmetric conductive film on the cathode electrode surface or from electrochemically developed insoluble insulating films or gas polarization films on the cathode electrode surface.
Since both the anode and cathode electrode possess inherent asymmetric conducting properties, the two electrodes are series-opposed with respect to their arrangement in the capacitor structure. When under the influence of an applied pulsating voltage, the electrodes charge and discharge alternately; that is, one electrode discharges as the other charges. As a consequence, the electrolyte between the electrodes remains at a negative potential toward the external electrodes throughout an alternating cycle. This differs from the charge-discharge function of two ordinary electrostatic capacitors connected in series, however, the law governing the admittance of the circuit remains the same: 1/C (device)=1/C (anode)+1/C (cathode) where C is capacitance. This relationship results in the condition that the charge transfer is limited by the smaller of the two capacitances in either arrangement.
In the design of electrolytic capacitors, particularly relating to capacitor rating, the design is established invariably from the design parameters of the anode. Therefore, the cathode capacitance should be made several orders of magnitude higher than the anode capacitance by suitable arrangement or treatment so as to be compatible with the anode design. Thus, the term 1/C (cathode) in the above relation would become small relative to the other terms and the device capacitance would become essentially equal to the anode capacitance. Ideally, the operating characteristics of the capacitor approach optimum stability as the cathode capacitance approaches infinity. This condition, of course, can be attained only approximately in practical design of capacitors.
Several methods are known to increase the capacitance of the cathode electrode in electrolytic, film-forming metal capacitors and they include;
(1) applying to the surface of the cathode electrode a layer of finely divided, substantially inert conductive material such as carbon or certain of the platinum metals or gold; when properly applied, these materials provide a very high cathode surface area necessary for cathode capacitance, or
(2) providing certain metal ions in the electrolyte which are capable of being electro-deposited on a cathode electrode of a suitable metal and dissolved therefrom in substantially the exact proportion of the current flowing back and forth across the cathode-electrolyte interface.
In system (1), it is thought that the current traversing the electrolyte-cathode junction discharges hydrogen or hydroxyl ions which are absorbed on the surface of the metal to yield a dielectric film. In system (2), it is believed the electrochemical discharge and dissolution of metal ions creates an ionic double layer in the electrolyte at the cathode electrode surface, and the space charge across this layer exhibits a high capacitance.
Since film-forming metal anode type capacitors are usually made as small as possible and the anode capacitance is therefore very high per device volume, the usual practice is to employ both methods of increasing the capacitance of the cathode electrode. However, applying the layer of finely divided conductive material to the surface of the cathode electrode is generally tedious and costly, especially when gold or platinum are utilized. The method generally involves the steps of cleaning the cathode electrode, applying a coat of masking material over a portion of the cathode electrode surface, etching the surface with an acid, rinsing, filling with plating solution, electroplating with a platinum anode, removing the anode and plating solution, rinsing, drying and removing the mask material.